Since their discovery, oxide superconductors based on copper oxides have been widely studied. A key property is that the materials exhibit superconductivity at high temperatures relative to their traditional metallic counterparts. In many applications, it is important for the superconducting oxides to be composed of substantially one phase and have critical current densities (J.sub.c) that are high.
One family of oxide superconductors includes bismuth-strontium-calcium-copper-oxide compositions such as Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8-x (BSCCO-2212; where x is a value that provides a T.sub.c of about 80K) and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-y (BSCCO-2223; where y is a value that provides a T.sub.c of at least 100K). Of particular interest are compositions where bismuth is partially substituted by dopants such as lead (that is, (Bi,Pb)SCCO)
The oxide materials, being ceramics, are generally brittle and are difficult to process and manipulate. Composites of oxide superconducting materials contained in metal matrices, or metal sheaths, have mechanical and electrical properties that are improved relative to the oxide superconductors alone. The composites can be prepared in elongated forms such as wires and tapes by processes, such as the well-known powder-in-tube (PIT) process, that typically include a number of stages.
In the PIT process, first, a powder of a precursor to a superconductor is prepared. The precursor can be a single material or a mixture of materials. Second, a metal container (for example, a tube, billet or grooved sheet) is filled with the precursor powder. The metal container serves as a matrix, constraining the superconductor. Third, the filled container is deformed in one or more iterations (with optional intermediate annealing steps) to reduce the cross sectional area of the container in a draft reduction step. A number of filled containers (filaments) can be combined and surrounded by another metal matrix to form a multifilament article. Finally, the material is subjected to one or more deformation and phase conversion heat treatment cycles which together form the desired oxide superconductor from the precursor and helps the oxide superconductor grains align and grow to form the textured superconductor article.
If the precursor powder is composed of one or more oxides, the process is known more specifically as oxide-powder-in-tube (OPIT) processing. See, for example, Rosner, et al., "Status of superconducting superconductors: Progress in improving transport critical current densities in superconducting Bi-2223 tapes and coils" (presented at the conference `Critical Currents in High Tc Superconductors`, Vienna, Austria, April, 1992), and Sandhage, et al., "The oxide-powder-in-tube method for producing high current density BSCCO superconductors", Journal of Metals, Vol. 43, No. 3, 1991, pp. 21-25, all of which are incorporated herein by reference.
A method of preparing BSCCO superconducting materials, particularly lead-doped BSCCO, is described in U.S. Ser. No. 08/467,033 filed Jun. 6, 1995 and entitled "Processing of (Bi,Pb)SCCO Superconductor in Wires and Tapes," and U.S. Ser. No. 08/331,184 filed Oct. 28, 1994 and entitled "Production and Processing of (Bi,Pb)SCCO Superconductors," both of which are incorporated herein by reference. In particular, the composition of the precursor powder is controlled to improve processing of the precursor powder. One feature of the BSCCO-2223 processing path described in this patent application is that tetragonal BSCCO-2212 (T-2212) is thermally converted to orthorhombic BSCCO-2212 (O-2212) prior to formation of BSCCO-2223. The phases can be distinguished by their X-ray diffraction patterns and lattice parameters.
Texturing helps to increase J.sub.c of the article. Texturing can be accomplished by deformation (deformation-induced texturing; DIT) or by phase conversion heat treatments which cause, for example, sintering or partial melting and regrowth of desired superconducting phases (reaction-induced texturing; RIT). Certain superconducting oxides, particularly BSCCO-2223 require precursor deformation for adequate performance. Density and texture of the product is increased in the article by sequential repetition of the deformation (D) and phase conversion heat treatment, or sintering (S), steps "n" times (nDS). Typical processes are 1DS to 5DS. Multiple low reduction deformation steps are described, for example, in Hikata, et al., U.S. Pat. No. 5,246,917, incorporated herein by reference. A single high reduction deformation step which can be used in 1DS processes is described in U.S. Ser. No. 08/468,089 filed Jun. 6, 1995 and entitled "Simplified Deformation-Sintering Process for Oxide Superconducting Articles ", incorporated herein by reference.
In all stages of processing, it is important to avoid the formation of undesirable secondary phases. The presence of secondary phases can disrupt DIT of the primary phase of the precursor powder, can lead to gas evolution and can induce melting during heat treatments.
The metal container, sheath, or matrix which holds the precursor powder prior to and during processing is typically composed of a noble metal such as silver, gold, platinum, and palladium, or an alloy substantially comprised of a noble metal. A noble metal is a metal which is substantially unreactive with the oxide superconductor or the precursor powder under processing and use conditions. Oxide dispersion strengthened (ODS) silver alloys have been used as matrix materials The silver alloy is exposed to an oxidizing atmosphere to generate the metal oxide dispersion in the matrix, thereby increasing the strength and hardness of the matrix. In general, when oxidation occurs through reaction with atmospheric oxygen, the outer portion of the article becomes harder than the center, decreasing the flexibility of the article. See, for example, Lay, U.S. Pat. No. 5,384,307, incorporated herein by reference. Lay co-oxidizes the matrix and the precursor during the step that forms the desired (final) oxide superconductor by heating the article in an oxidizing atmosphere of about 3 to 14 volume percent oxygen. One problem of this process is that the superconductor can be "poisoned" as the matrix and precursor compete for oxygen. In practice, small amounts of poisoning can drastically reduce J.sub.c. Thus, many ODS silver-superconductor composites have lowered current carrying capabilities. U.S. Ser. No. 08/626,130 filed Apr. 5, 1996 and entitled "Oxygen Dispersion Hardened Silver Sheathed Superconductor Composites ", incorporated herein by reference, describes one solution, but it requires metallic alloy precursors and cannot be practiced with oxide precursors. Since OPIT-based composites generally have good J.sub.c performance, it is desirable to find a readily manufacturable, non-poisoning OPIT processing route to high performing ODS superconducting composites.